In my past life, I dealt with occult things, specifically sigils.
I have since reverted and been absolved of all related sins, but yesterday came across an old journal with some sigils in it from that time.
Do I need to do anything in particular with the journal, or can I just shred it?
First, I am so glad that you got yourself to confession and gave up all that dangerous demonic stuff.
Occult involvement can open up the door to demons, who then claim the right to attach to objects, places and people. Their bond has to be broken.
In the case of possible attachment of demons to that journal, if you can, find a priest who is able to read Title XI, ch. 3 of the Rituale Romanum over the object before burning it. Then the ashes should be thrown into running water, like a river.
If some object can’t be burnt, as in the case of metal or stone amulets, etc., break the thing so that its form is gone, and then throw the parts into a river.